In present day medical practice, hemodialysis is the standard therapy for treating ESRD. This therapy involves dialyzing the patient's blood several times a week. During treatment, the patient's vascular system is connected to a hemodialysis machine for sessions lasting several hours. This connection forms a blood circuit whereby blood is drawn from the patient through a needle connected to a flexible blood line, cycled through a hemodialysis machine that removes waste products including water, urea, and other impurities from the blood, and returned to the patient via a second blood line and needle. As used herein, reversing the direction of flow in the blood circuit means drawing blood through the needle which had previously been used to return blood to the patient and returning blood through the needle which had previously been used to draw blood from the patient without changing the direction that blood circulates through the hemodialysis machine. This change is also referred to herein as flow reversal or flow reversal in the patient portion of the blood circuit.
Traditionally, access to the patient's blood stream has been provided by an arterio/venous ("A/V") fistula or by a polytetrafluoroethylene ("PTFE") graft. An A/V fistula is a surgical construct joining an artery to a vein. The shunting of blood from an artery to a vein increases pressure on the vein, which pressure enlarges its diameter and thickens its walls. A fully developed fistula can be punctured with needles to access the patient's blood system. A PTFE graft is an artificial blood vessel used to connect the artery to the vein. The material used for the graft is suitable for puncturing with needles to achieve the necessary access to the patient's blood system. A third method of obtaining access for hemodialysis is to use percutaneous catheters which allow blood to be withdrawn from one lumen and returned by a second lumen. New methods of accessing the patient's blood stream are also being developed. The Dialock.TM. system accesses the superior vena cava through one or more catheters connected to an implanted prosthesis. Needles are inserted into the prosthesis to effect access to the patient's blood stream.
It is advantageous to operate hemodialysis therapy with blood flows at the highest rates possible while avoiding damage to the blood cells. The device facilitates selection of the best flow path at each dialysis session giving the highest flow rate for the maximum allowed negative pressure.
One of the difficulties that can arise in chronic hemodialysis is maintaining adequate blood flow during treatment sessions. When flow rates decrease significantly during a session, the attendant could in many cases restore adequate flow by switching the blood lines. In current practice, the attendant must usually turn off the hemodialysis machine. This process lengthens the dialysis session while the machine is primed and restarted. In addition, switching the blood lines involves disconnecting the lines, which can cause bleeding and allow air to enter the lines.
Disconnecting the lines also breaks the microbe barrier, increasing the possibility of infection. Thus, blood lines are typically not switched during hemodialysis treatment absent extreme need, despite therapeutic benefits that could accrue from periodically reversing the direction of flow in the patient portion of the blood circuit. If flow direction could be reversed easily during operation, then the best path, the one which yields lowest negative pressure reading, could be easily selected and reselected as the session proceeds.
A further situation which often arises is that the flow resistance in each of the catheters in the patient is different. The pressure drop or resistance that blood cells can be subjected to without damage is higher when the blood is at pressures greater than atmospheric (return side of the dialysis machine) than when the cellular components of blood are subjected to negative pressures. Absence of ability to reverse the direction of blood flow easily can result in lower flow rates, larger negative pressures, and increased possibility of damage to blood cells.
Another difficulty that often arises with chronic hemodialysis is the possibility that the patient will develop a thrombus or blood clot that partially or wholly occludes a vascular access created by a fistula or vascular graft. When a fistula or graft becomes blocked, surgery is frequently needed to restore the venous access to a useful condition or replace the access site. A balloon angioplasty may be used to enlarge the lumen of the fistula or graft and prevent the immediate formation of thrombosis, thereby extending the life of the access. When a site can no longer be restored, it must be replaced. Replacing an access is a serious matter because patients have only a limited number of access sites for A/V fistulas and PTFE grafts.
Even when dialysis is being performed using catheters or Dialock.TM., flow can become impaired. Flow impairment often occurs because of the formation of a fibrin sheath or a clot at the tip of a catheter lumen. In some instances, this sheath or clot can completely block flow if the catheter lumen is being used for withdrawing blood from the patient. (A clot at the tip of a catheter lumen usually does not interfere with blood return.) In many cases, therefore, the impairment could be corrected or mitigated by reversing the direction of blood flow during a dialysis. Moreover, maintaining a high rate of blood flow throughout the session slows the development of thrombi and potentially prolongs the useful life of the access. Furthermore, there is a possibility that reversing flow direction during a hemodialysis session or between successive sessions will reduce the occurrence of fibrin sheath formation.
It is known to use spool valves or three-way valves with syringes, and it has been suggested that these types of valves could accomplish flow reversal. However, these valves were not intended to accommodate high rates of blood flow. Specifically, the fluid flowing through valves of these types is required to make abrupt turns, creating turbulence in the blood flow. These valves also contain abrupt enlargements and restrictions which also engender turbulent flow. Turbulence reduces the blood flow rate and promotes the formation of thrombi. These valves have the additional prohibition that they contain spaces that could accumulate stagnant blood.
A number of inventions relate to reversing the direction of flow in connection with hemodialysis. U.S. Pat. No. 5,605,630 discloses, in pertinent part, a "blood flow direction changeover means that is attached to the blood circuit for changing over the direction of the blood flow through the dialyzer from one direction to the other." Similarly, U.S. Pat. No. 4,885,087 concerns the means of changing the directional flow through a mass transfer chamber such as a hemodialysis machine. U.S. Pat. No. 4,324,662 describes a "flow reversing valve system . . . positioned in the flow system for cooperation with the dialyzer to selectively control the direction of dialysis solution flow within the dialyzer in either a first direction or a second reverse direction." Each of these inventions concerns reversing or alternating flow into a transfer chamber such as a dialyzer. The inventions are unsuited to changing the flow of blood to and from the patient, because they are not concerned with maintaining laminar flow in the blood circuit. Even if these inventions were adapted to reverse the direction of flow in the patient portion of the blood circuit, none teaches a bypass function to ensure that the dialysis machine can continue to operate while the direction of blood in the lines is being reversed.
Several other inventions relate peripherally to the subject matter of the instant invention. U.S. Pat. No. 4,586,920 discloses an inflow/outflow directional valve acting in combination with a bypass valve used in a continuous flow peritoneal dialysis system. However, the so-called directional valve does not change the direction of flow through the peritoneal catheters--the inlet and outlet catheters remain distinct during dialysis. U.S. Pat. No. 4,898,669 describes a rotating valve employed in vascular access having two positions: the first connecting the blood system inflow and outflow with two external ports and the second position acting as a bypass to short circuit blood system inflow and outflow, and to allow flushing of the external ports. This device does not teach reversing flow in the patient portion of the blood circuit, particularly as it might be combined with its bypass function.
A number of inventions manipulate the passage of blood through valves and manifolds. U.S. Pat. No. 4,946,434 "provides a plurality of sterile paths for directing the flow of fluids." Similarly, U.S. Pat. No. 4,821,996 discloses a "multi-position rotary valve system usable with a disposable fluid transfer set . . . " These and other similar inventions rely on clamping to selectively open and shut flow paths. Clamping is unsuitable in the current circumstances because of the need to maintain blood flow through the hemodialysis machine during flow reversal operations. Any device which necessitates restarting the hemodialysis machine would not be useful for routinely reversing flow in the patient portion of the blood circuit.
Accordingly, an object of this invention is to provide for the easy and convenient selection of which needle or catheter will be used to draw blood and which will be used to return blood at any particular time during hemodialysis treatment sessions. Another object of this invention is to have a device that is compatible with high rates of flow in dialysis methods such as Dialock.TM. which utilize catheters. Yet another object of this invention is to accomplish the flow reversal function while minimizing the amount of turbulence associated with blood flow through the device. Still another object of the invention is to provide a device that is safe to use. A further object of the invention is to minimize stagnant flow regions in the device. Still another object of this invention is to provide a device that is easily added to existing hemodialysis set ups and treatment programs. Still another object of this invention is to provide a low cost, easily manufactured, sterile disposable device compatible with the rest of the blood circuit.